Allen Bradley 1791-8BR Block I/O Module
The Allen Bradley 1791-8BR is a block-style input/output module from Rockwell Automation, designed for remote or local I/O applications in PLC-5 and SLC 500 systems. It combines eight discrete input points and eight relay output points in a single self-contained unit, reducing wiring and panel space while simplifying machine-side I/O connection.
Input and Output Specifications
The 1791-8BR features 8 inputs that accept both AC and DC voltages over a wide range (85-132V AC, 10-30V DC), providing universal compatibility with various sensors and switches. Inputs are optically isolated from the backplane and logic side, offering protection against surges and noise. Each input has an indicator LED for status visualization. The inputs are arranged in a common return configuration, simplifying field wiring.
The relay output section provides 8 independent form A (normally open) contacts. Each relay can switch up to 2A at 120V AC or 24V DC (resistive load) with a maximum inrush current of 5A. Outputs are individually isolated, allowing mixing of AC and DC loads on the same module. Relay status LEDs indicate when each output is energized. Outputs are protected by replaceable fuses for overload prevention.
Communication and Integration
The 1791-8BR connects to the controller via a single ribbon cable to a 1791-series adapter or directly to a remote I/O scanner using Allen Bradley’s proprietary RIO protocol. It supports block transfer for configuration and diagnostics. The module’s address is set via DIP switches, and it occupies 16 input and 16 output words in the scanner’s image table. Data is updated each scan cycle, ensuring real-time control.
For system integration, the 1791-8BR is compatible with PLC-5, SLC 5/04, and ControlLogix using appropriate RIO communication modules. It can be mounted up to 1000 feet from the scanner using Belden 9463 cable, enabling distributed I/O across large machinery or processing lines. The module operates in temperatures from 0 to 60°C and is rated NEMA 12 for protection against dust and dripping water.
Applications
Typical applications for the Allen Bradley 1791-8BR include material handling systems, packaging equipment, conveyor control, small assembly stations, and industrial ovens. Its mixed input type (AC/DC) and relay outputs make it suitable for controlling contactors, solenoids, indicator lights, and small motors. The block design reduces installation time with pre-wired connectors compared to traditional rack-based I/O.
Maintenance is simplified with modular design, fault indicators, and removable terminal strips. The relay outputs are replaceable as a complete assembly. The 1791-8BR has been widely deployed in automotive, food processing, and pharmaceutical industries due to its reliability and simplicity.
Electrical and Mechanical Characteristics
The unit requires 24V DC power supply (300 mA typical) and provides surge suppression for output relays. Dimensions are 5.5 x 7.0 x 3.5 inches, and weight is approximately 2 pounds. It is certified by UL, CSA, and CE for global use. The 1791-8BR continues to be a cost-effective solution for machine builders requiring flexible, rugged, and easy-to-install distributed I/O.
